If Crikey was still active he’d tell you about how all you need is a wrist sundial and how he ran his first marathon wearing a granite one he’d had to carve that very morning, and your generation don’t know your born, all skill compensators, just go for a run, etc, etc.
In other news, I have a Fenix 3, which seems accurate in terms of GPS and HR (with a strap). My missus has a forerunner with the built in HRM, which is good but quite often has mad spikes,and the occasional inaccurate GPS data.